(1) Field of the Invention
The present invention relates to a multilayer printed wiring board having a multilayer wiring pattern and a process of producing same, and more particularly, to a multilayer printed wiring board which permits the formation of high-density wiring patterns and a production process therefor.
(2) Description of the Related Art
In recent years, MCM (Multi Chip Module) having a plurality of bare-chip LSIs mounted at high density on a multilayer wiring board has come to be widely used. MCM is used in various types of OA equipment, mobile communication equipment, industrial equipment, etc., such as in notebook computers and mobile phones, and has been greatly contributing to the reduction in size and weight of such equipment In the field of notebook computers, mobile phones and the like, further reduction of their size and weight is expected in future as well, and thus there will inevitably be a demand for corresponding miniaturization of MCM.
Requisites for the miniaturization of MCM include reduction in size of LSIs to be mounted, high-density formation of wiring patterns, etc. In the case of a flip chip used as an LSI, its pad pitch is said to become as small as 0.07 mm or thereabout in future. Accordingly, a multilayer wiring board also, on which flip chips are mounted, needs to be formed with high-density wiring patterns matching such fine pad patterns.
As such multilayer wiring boards, a ceramic wiring board using a ceramic material and a buildup wiring board using a glass-reinforced epoxy resin etc. are generally known. A ceramic wiring board is produced using a green sheet, and a through hole is formed in the green sheet by punching. A wiring pattern is printed on the green sheet by using an electrically conductive ink. A large number of green sheets thus formed with the through hole and the wiring pattern are stacked up and are baked at high temperature under high pressure, thereby forming a wiring board having layered wiring patterns. To produce a buildup wiring board, on the other hand, a copper-clad, glass-reinforced epoxy resin is used as a starting material. After a through hole is formed using a drill, a conductive layer is formed on the inner wall of the through hole by plating to achieve conductor connection between the opposite surfaces of the copper-clad, glass-reinforced epoxy resin. Subsequently, a wiring pattern (hereinafter core layer) is formed, and an organic insulating layer (hereinafter buildup layer) is formed on one or both surfaces of the material having the wiring formed thereon. Then, the buildup layer alone is removed using a laser or by etching at locations corresponding to interlayer connecting portions, and the individual layers are interconnected by means of plating, thereby forming a multilayer wiring board.
In the case of the ceramic wiring board, however, since the wiring pattern is printed on a green sheet, it is difficult to form a high-density wiring pattern.
Further, in the ceramic wiring board, a through hole is formed by punching, and thus it is difficult to form a small-diameter through hole. As a result, the land has an increased width corresponding to the diameter of the through hole, also making it difficult to enhance the density of the wiring pattern.
On the other hand, in the case of the buildup wiring board, the coefficient of thermal expansion (16 ppm/° C.) of the plated copper formed between the copper foil constituting the core layer and the glass-reinforced epoxy resin greatly differs from the coefficient of thermal expansion (80 ppm/° C.) of the glass-reinforced epoxy resin in the thickness direction thereof. In order to eliminate inconvenience such as wiring disconnection caused by the difference in the coefficient of thermal expansion, therefore, the thickness of the plated copper must be 20 μm or more. Further, the underside of the copper foil constituting the core layer is roughened in the order of 3 to 5 μm to ensure satisfactory adhesion strength when the copper foil is laminated on the resin, and therefore, the roughening thickness of 3 to 5 μm must also be allowed for. Thus, even if an ultra-thin copper foil is used, the total thickness of the copper foil, including the plated copper and the roughening thickness, becomes 35 μm or more. When such a thick copper film is etched, the amount of side etching tends to increase, giving rise to a problem that the wiring of the core layer cannot be made fine.
Further, in the buildup wiring board, a through hole is formed by drilling, and thus it is difficult to form a through hole with a diameter of 0.3 mm or less. As a result, the land has an increased width corresponding to the diameter of the through hole, also making it difficult to enhance the density of the wiring pattern.